On the '90 the serp belt is tensioned by the power steering pump. There are about five bolts you have to loosen (not remove, just loosen), a couple of which are on the front of the pump bracket and accessible from on top, and a couple on the back that are more easily accessed from beneath or with the air box removed.
THEN there is a tensioner screw that you back off to allow the pump to swaing in. This is also more accessible with the air box removed.