What am I missing here? If they're not visible from the outside of the cabinet front or side, why do they need to be fixed? Who care about a hole in the back of a sink cabinet that no one is going to see.

Mice highway? They'll chew through what they want to or simply find another way to get to where they want to get. You need to keep them out the house, no just out from under the cabinet.

I understand that exterior entry points are the wiser bet, but we don't know where they're coming in - we've looked. This is, however, the only place in the house where they leave evidence of being. We want to seal the gaps to at least try and prevent this entry point. That's my question.

I am not sure how effective it is but I think some people plug rat holes or potential rat holes with steel wool.

Yes, but I also understand that steel wool is not recommended against copper pipes due to corrosion.

I don't know anything about anything, but I just had reason to buy a can of Great Stuff foam sealants, and the picture on the can shows it being used for this exact type of hole around a pipe.

If you are concerned about corrosion of steel wool, use copper wool.

This was my first thought. We'll need to reroute the power cord pictured in the one photo, as well, so we wouldn't use foam until we moved it. No idea how to do that, but know we need to do it.

I would find some scrap cement board, plywood or other thin wood, scribe an opening for the pipe & cable, and fit it around the pipes/glue/fasten in place. I would also use some great stuff, maybe stuff a nylon bath pouf or two in there. Mice can and will chew through wood, but perhaps the combination of materials will deter them.

Or you could get wood filler (the kind for repairing areas of wood rot in trim, door casings etc. and see if you can fill in the gaps with it instead of the board.

"I understand that exterior entry points are the wiser bet, but we don't know where they're coming in - we've looked."

Hire a good exterminator. They're worth it. I had a terrible mouse problem in the attic, an infestation not just a few. Exterminator came out -- whatever bait blocks he used (52 of them!) were no joke, no mice after about two weeks.

My 2 cents...

First, you need to get rid of the current mouse problem. Things that will keep mice out will not necessarily move them out. Plugging a hole isn't going to help once they are in the house and arguably isn't even effective on the exterior. A mouse only needs a dime sized hole for entry and can create one if need be.

So get a humane trap, load it with peanut butter in an area where there is evidence of mice, and then either release the mouse elsewhere or dispose of it another way.

Next, remove the attraction. We live on some acreage and if we are not careful we can get mice. Mostly we try not to attract mice by properly sealing food items. E.g. once cereal is opened it goes into a cereal container rather than staying in the box. It is pretty effective.

Finally, deter mice from coming in. A cat will typically deter mice from entering an area, but cats aren't for everyone. Sealing exterior holes will help but it is difficult to find and seal all dime sized holes and mice will create more in many structures. Use cinnamon sticks strategically placed around your home. Mice have a strong sense of smell and are often deterred by powerful scents. Cinnamon and peppermint oil are two things that work which are not offensive to most people. You can also spritz your house with vinegar which has the added benefit of also keeping in-laws away, but that is a problem for some.

Good luck and have fun.

Buy some very cheap floor tile, that looks nice, and make sure it is the type you glue on. Do the entire back of the cabinet, and cut carefully, using two pieces around your pipe. Once its all on. Caulk around the smaller gap that will still be there, and over the top of that use the 2 inch wide flashing tape. Tape it tight. It works. I did that in a friends house. Worked great, and looks much nicer too.

Slide some armored cable around that exposed Romex please. Before you plug the hole.

Thank you. I have no idea how to rectify this cable situation. I don't even know where it's going/what's in the wall. I don't know how to slide armored cable onto it if I can't access the end. It's all a mess.

Their friend who installed the cabinetry missed a lot of stuff. A lot. There was a lot around the whole house that went unaddressed for many years. This is why I KNOW an exterminator/eliminating access is the best plan - as so many people have pointed out - but it's a huge task and one we aren't prepared for until we fix a lot of other things. This is why I'm trying to fix one thing at a time. Plus, we live in an area where we've tried for over a year to get a handyman to help fix other things. There's just no one available to help with things we cannot do or do not feel comfortable doing on our own.

And to those suggesting we get a cat, we have a cat. Cat doesn't go under the sink or behind the dishwasher and cabinet drawers.

The electric line that runs through there, requires an electician, not a handyman. Make sure its a licensed electrician.

And to those suggesting we get a cat, we have a cat. Cat doesn't go under the sink or behind the dishwasher and cabinet drawers.

The scent of a cat is a deterrent, it doesn't have to go under the sink or behind the dishwasher and cabinet drawers to do that. However, a deterrent alone isn't going to overcome other problems.

For a last resort put out DeCon bait to get rid of the mice.

I inherited a mouse problem in my last house. The mice lived in the walls, chewing their way throught the wall insulation to make a nest at several electrical outlets, because that's where it was warm. You might want to start by setting up traps under the sink, while you figure out a more permanent solution. Don't use poisons, because the mice will often flee out of the house when they are ill and then be preyed upon by wildlife, such as owls, which in turn will kill the owls. I never solved my mouse problem in that former house, because there were so many holes between the foundation walls and the upper walls that I never plugged, And so, every fall I had to set up traps to catch the infiltrating field mice. It was manageable and they no longer lived in the walls once I removed the bird seed that the previous owners stored in the attached garage.

I have no idea how to rectify this cable situation. I don't even know where it's going/what's in the wall. I don't know how to slide armored cable onto it if I can't access the end.

Probably new feed for a dishwasher or garbage disposal. If so, easy enough to disconnect at either a switch for the disposal or a receptacle box. Or at the dishwasher if the DW is hard wired. After you shut down the breaker to that circuit, of course. ;-)

Buy steel wool in paint department and poke in hole. cover with pipe cover like this;

https://www.homedepot.com/p/OATEY-3-4-in-Chrome-Plated-Steel-Iron-Pipe-Size-Split-Flange-Escutcheon-Plate-HDC5352C/316622083?ITC=AUC-142525-23-12140

They can't chew through either.