While attempting to change the thermostat on my 87 Corolla, the bottom bolt broke off of the housing. What now? How do I remove the back half of the housing to either a) drill out the bolt or b) replace the part? What is the back half of the thermostat housing called? The parts store catalogs the top half as the t-stat housing & doesn't have a clue what the other half is. Any suggestions are very appreciated!