is money or space an issue?
if not, water cooling holds some possiblities.
first off, AC is a special case, but generally you want to be careful about cooling anything colder than ambient, any vapor in the air will condense on anything colder than it. AC dehumidifies the air, but if humid air from somewhere else hits it... not good.
same goes for water cooling though. if you are water cooling, and the trunk is filled with hot humid air, it will condense, you can combat this with neoprene though.
whatever you do remember rule 1. the heat has to go somewhere.
I guess the best place is outside, as outside will be cooler than inside...
to be stationary and still cool, you need to use a fan. multiple fans infact to have any chance of cooling a 100+degree area and venting to a 90 degree area. also going to need surface area.
Im thinking 2-3 fans, straped to a large automotive radiator. but still gonna take a few minutes of circulation to cool it.
I think generally you have 2 areas both of which are too hot for a PC, no where to vent. you need to reduce the heat put out. somehow.