I agree with 4crawler, I had a similar problem, spent weeks trying to trace it and in the end spent an hour putting in a relay and haven't had a problem since. By the way, if/when you go the relay route the standard 4 pin universal relays they sell at napa fit perfectly in the fusebox in the engine compartment, makes a clean looking installation.

86 4runner, pro comp AAL, Kayline Soft top, 31's but all stock