well im seeing 12.8v at the battery and only 10.2v at the amp. something is wrong im going to put on a new fuse and holder but how do i test the ground?

To test the ground, I'd first test the battery and chassy connection, to make sure the chassy is seeing the full ground. Do this with your ohm-reading function on the multi-meter. It should read no resistance. Then, take a similar reading between your amp'* ground wire (disconnected) and the chassy from anywhere in the back. It should also ready no resistance. If both those look okay, take a length of wire (ANY speaker wire works fine) and run it from battery negative terminal to trunk, and do a reading between it and the ground wire that is now disconnected from your amp. All should show no resistance, otherwise, you've more than likely got a poor connection.

When I do connections I usually use a fresh length of wire freshly stripped at the end, and screw it to whatever after rubbing a little of that corrosion stopping dielectric stuff on the end. Then if it'* done right the first time, it'll be the only time you'll need to do it.

Hope you get the problem figured out.

Good advice. Also, I'll be the one to ask the simple question. The amp ground wire is secured to a bare piece of metal in the trunk with a star washer, right?

Apply both to your tounge and...

Oh wait.

Multimeter works good.

Yup, the star washer if it'* what I'm thinking is a good way to go. I've never had any problems with regular flat washers either, so whatever is most convenient or laying around

well my amp isnt in my cuz my trunk leaks and water and electronics dont mix. so its under my seat and for a ground i used a ground thats already their. well i put in a new fuse and fuse holder and so far its fine 13.8 v at the amp.

I'm glad that you found the problem. Does that fuse holder say "MADE IN TAIWAN" anywhere?