Jeep Cherokee - overheating, and what does the red "EXH" light mean?

Hi all, I have a friend with a Jeep Cherokee (about a 96 model, i think) with the 4.0 litre injected engine, auto. It has a red light down on the dash marked "EXH" which is now permanently on. Can anyone tell me what it means? I should add that this is an American model, LHD (we are in Vanuatu). It has also started overheating - he says the light came on about the same time. I've checked for water in the oil, funny coloured exhaust, cream in the oil cap, but no signs there - but the overflow bottle regularly overflows. This also confuses me, as it doesn't seem to have a separate radiator cap, everything seems to actually circulate through the expansion bottle. How does that work with pressure - is that bottle actually pressurised, and if so where is it supposed to vent, as I can't see anything anywhere that looks like a means of pressure relief?

Thanks, Tim Z