I was just wondering what the gain control does under my stereo (95 SSE)

Also, does it still work with a non-stock receiver? I couldnt tell a difference with stock tape deck or with new CD player...seems worthless

Its kinda like a bass booster for the stock system only. Won't do you any good with an aftermarket deck, unless you rewire it to like a subwoofer gain or something like that.

if you didnt connect it and run the extra wire you have disabled the amp.....it is a VERY noticable difference when a aftermarket deck is installed....i would search on here on how to make the Aftermarket HU work with the Gain amp....its worth it

If you wired your deck directly up to the stock harness, it should work. There should be a small amp hanging underneath the radio. In stock form, it increases signal to the woofer portion of the rear 6x9'*.