i own a 96 base 4x2 4runner,i have my 4 channel amp under my driver seat for 9 years, and it was a phoenix gold sapphire series amp sa1.4x, never had a problem with heat, cheap amps heat up faster, the amp died last summer since it was juss to old, so i upgraded to a jl audio 300/4 and it stays cool there, nothing wrong with mountin an amp under the driver seat. do not install capacitor, it will do more harm, i suggest doing the big 3 upgrade with 4 gauge or larger and it will stop the dimming, if that dosent help, a capacitor with that big 3 would help, but i dont reccomend using caps. other wise upgrade to a better battery, like a optima deep cycle, or kenitik or oddessy batteries and itll even help u out more. i reccomend u to do a 1 gauge big 3 upgrade on ur car and it will stop the dimming