You need to put some punctuation in that huge sentence, otherwise it can be read at least three different ways.

Start by measuring the battery voltage. You can also do this test with a test light, but turn on the head lights so some current is trying to leave the battery. Move the probes from the battery posts to the cable clamps, then down the line to the next place you can measure. You're looking for the first place voltage drops significantly. That will be the point of the bad connection. In particular, assuming the head lights are dim, follow the smaller positive battery wire to the under-hood fuse box and the smaller negative battery wire to the body. Be sure both of those connections are tight.