Mine has (Standard GS300, Auto 1995). Look at the silver metal heater water pipes near the passenger side firewall in the engine compartment. There is one sticking vertically up with a grey plastic cap with the letter L on it. Undo it and inside there is a pin valve (like those on tyres). Press on it with a blunt object and air gushes out and you are done; my heater was not working but now works a treat after I did this 10 second job.