This is not an issue just with Isuzu's. My Mazda 3 does the same thing when you switch it to defrost without the ac light coming on. Someone with allot of time on their hands came up with a cool fix that makes it work like we want it to, compressor only kicks on when you hit the ac button, but it entails tearing down the whole dash and some minor fabricating and would effect my extended factory warranty. If you are willing to tear apart your dash I bet you could install a similar fix.

98 Rodeo
00 4Runner

--I swear its just over the next pass !!--

--"Courage is something you can't be afraid to have." - Frank Burns