I'm not sure how much stuff is in the way on a Viggen, but on an n.a. 2.3 it's very easy. The thermostat housing is held on by two (13mm?) bolts. Since they are tight, I'd recommend a 6 mm socket to avoid stripping them. If you siphon the coolant out of the reservoir, you won't have all that much dripping out when you take open the thermostat housing. Depending on when your last coolant replacement was, you could filter the siphoned coolant through a coffee filter or something, OR just drain, flush, and refill the whole system if you're due. If you are due, there should be a few recent threads on the flush and fill procedure, along with various thoughts on the best antifreeze to use.