A piezo has only two wires, there is not much to look into. In case of the rim piezo, one wire goes to the sleeve(ground), another goes to the ring. If you don't get a signal, the problem is one of these:
1. The ground wire is not connected to the sleeve or poorly soldered.
2. The signal wire is not connected to the ring or poorly soldered.
3. A short between the ground and the signal wires.
4. The rim piezo is broken.
the thing is that i have try whit diferent piezos, and diferent jack , and diferent cable.
don't worry, i will buy new calbes and piezos and jack, maybe this is the problem, i don't know, i hop so.
any way, tahnkyou very much for helping,
i will tel you when i find the solution
realy thankyou very much