Chris
Front and rear speakers should be set to HPF any subs set to LPF
Firstly recheck your work ( some cheap elecy tapes peel off, Wires rubbing on an edge) Sounds like a shorted speaker or maybe a ground loop. If not you should still able to be fault find.
1. Unplug the RCA's to the AMP and then turn back on, does the tone contunue?, does the Protection light come on?
If neither, the problems probably in the RCA's (loosing ground) or maybe the Deck. Check indash wiring and Voltage to the Deck
2. Remove the Speaker wires from the AMP and turn back on, If the Protection light and the tone continue check the voltage at the AMP, otherwise the AMP may be toast
3. Test the Speakers, grab an old 9V ( 3/4 flat ones are best), stick it on each speakers wires in turn, you should get a crackle/pop out of the speaker if its OK
PS. Do you remember which wires you resoldered? (Amp to Speaker?)
Cheers
Bundy