i have a 1984 toyota corolla ke70. ive put in a new sound system. head deck, 2-6x9 speakers, 2-4inch speakers in the front. when ever i put on the blinkers or the head lights, ther is a skip in the music and the head deaks lights go out for a quick flick. it is very annoying. i was woundering if it is the alternator cause the lights also get brighter and darker. ive got a volt gauge and it goes up to 18 volts then bak down to 13/14 volts. any one got any ideas?

Alternator shitted out, or bad earth somewhere under the dash. Run the headunit earth straight to the chassis, then in the engine bay go from the negative battery terminal to the chassis with a length of 2 gauge. If this is a no-go, take your alternator to an auto eleccy and say you want it rebuild with Falcoon 60A or even VLT 80A internals. Might also be an idea to replace the wires between your alternator, voltage regulator and battery with something a bit heavier too.

an update. it still spikes upto 18 volts but then drops below 11 volts. also wen i rev it, the lights get brighter. can any one tell my wats happening.

Voltage regulator or alternator.

I would agree with the random hero either your alternator or voltage regulator has crapped itself. an auto electrician would be able to run some tests for you to narrow it down