i've installed my wife's 2k 4runner stock radio in my '88 and have replaced that with a scion head unit - both are double din sized.
although, neither are navigation.
i know at least one, maybe more?, who have installed a double din nav unit in a first gen 4runner.
what are your concerns?
i will tell you that both the 2k stock unit and scion unit were plug-n-play.
both needed a little help to fully seat into the radio cavity.
i don't believe i was able to use all four mounting screws... was only able to use either the top two or bottom two.
for the scion unit i have to cut the radio bezel for it fit.
joel