Heater won't turn off

I have an old Jura Capresso J5 that I've broken down and reassembled multiple times. Somewhere along the line I must have failed to connect a wire correctly. The unit brews properly but the heating element never turns off. I can work around this by unplugging it or turning it off however this is a safety hazard if someone forgets to do so. Any ideas what I might have done wrong?