Definitely sounds like something is shorting out.
The wires you're looking for are:
(Coming out of the head unit *high side*)
Right front: + GRAY
Right front: - GRAY/BLACK
Did you cut into the wiring harness or use an aftermarket connector like this?
Metra 70-1761 Radio Wiring Harness For Toyota 87-Up Power 4 Speaker
Amazon.com
If the speaker and connections there check out fine as well as to the amp then it's most likely a bad connection coming out of the head unit. I'd pull the harness where it connects to the back of the hu if it's a "quick disconnect" and make sure no pins have backed out of the connector. Then make sure it seats solidly back to the hu. I'd also check the 10 pin connector (if applicable) where it connects to your vehicles stereo wiring harness and check for any pins backed out. Other than that, if you used butt connectors that would be the most likely culprit.