My local mechanic gave me this trick. I was having trouble getting my thermostat and o-ring to stay put while trying to re-install them on my 1.5. Awkward angle, o-ring wants to walk off if you don't nail it right away, I'm sure you've all ben there. So disconnect your top radiator hose at the radiator and stick a shop vac hose in it - the shop vac will hold the thermostat in place, and a little vaseline or RTV will hold the o-ring onto the plastic elbow. Now you can take your time. I can't believe I didn't think of that -
Greg