Thermostat and housing is all one piece on the Gti mate if thermostat comes on its own you have the wrong part, changed mine couple months ago
Had a row in vw and tps as they kept trying to sell me a different part number because thats what was listed on there system!
Told him the system is wrong and just fooking order the part number ive asked you for mongy
Part number - 06F 121 111 F
Or 06F 121 111 H = edition 30 thermostat number both are around £35