are teh coils wired out of phase?

 

i dont see how they could be i havent opened the sub box or changed anything...just took it out of one car and put it into another..

 

I will assume the sub isnt dieing so start isolating possible problems with the amp. Unlikely a problem but check the power & ground to be sure. If you have a distro block w/ fuses check to see if a fuse is damaged but still letting current through. Double check your inputs to the amp, (from the hu) does your hu have a rca sub out & are you using it? is it turned up/on @ the deck? (simple things that can be overlooked) could be bad rca's, you could try to daisy chain the signal from the 4ch amp to see if it is infact a problem with the amp or hu or wires inbetween. Just start switching things one at a time to see what happens. Start trial & error troubleshooting.
Or just take it to a shop.

 

Same amp in the old car too?

Same deck in the old car too? If not, what deck is it now?

 

same deck... alpine...i didnt change anything just the wiring...same speakers everything.