that hosing down stuff, even if you cover up all your electronics, ull probably get some water in there, and consering your arent supposed to go to a carwash with an e36 b/c they are notorious for getting water into the ECU, i would just use a rag and same engine decreaser, but wipe it off, dont hose it off(wipe it off with anything, including armor all wipes, like death said)
also, WD40 is a neat trick, it doesnt clean it, but it makes it look amazing