A) element on the lighter is bad/ lighter needs replaced

B) it got unhooked/unplugged when the amp was being hooked up.

C) you didn't check the fuses under the hood, or missed one

D) something else

if it's A just buy a new lighter, simple to replace (on most cars, i don't know what you have

if it's B take the stereo out and check the wiring with a light tester

if it's C, then the answer's simple

if it's D, then I can't really help you, sorry