Don't disassemble the lamps unless you really have to.
First test: flick on the turn signal, and use a long wire or clip lead. Touch one end to the bumper, the other end to the chrome lamp housing. If it suddenly starts working, it's a bad ground.
I asked someone, while he had my car on the lift, to tighten it. Next thing you know, he was disassembling the indicator, then found nothing, then couldn't get it back together. Before long five people were on it, and ended up breaking the lamp. I went to an auto parts place and found a lamp, but it's dime -- and it's still not totally together -- and it still lights up intermittently because nobody ever DID tighten the mount!
I might end up buying a new set, and doing it all myself . . .