You can get a decent multimeter for about 15-20$ on amazon.
You are very welcome.
Here is the one I got, it does not come with the alligator clips you will use a good bit, but they are only a few bucks more.
Here is the one I bought a while back, great meter for 20.00
If you get that I'd get these with it.
If you end up finding out the channels are blown, and you want to replace it I may be able to help you find what you need.
I'd get a good aftermarket deck if it were me, unless I wanted to keep that original look.