I'm not sure how the 4runnas are wired from the factory but the problem with the stereo wiring maybe to do with the dimmer wire. On some factory stereos theres a wire thats live when the lights are turned on to either illuminate the knobs or dim the lights if they're too bright. ALOT of my mates have made the same mistake when wiring up stereos. On some vehicles (toyotas included I think) the illumination wire thats live when the lights are on is black or black with a dark stripe.

So people sometimes run their stereo wiring up to this black wire thinking it's an earth and all is good until you turn your lights on. Might be worth checking the earth the stereo uses <img src="/forums/images/graemlins/cheers.gif" alt="" />