The best way to troubleshoot will be to break out the meter. I'm guessing/hoping you've got a relay in there, right? 1st see if you're getting the 12v from the battery at the relay. Then see if you're getting a 12v signal from the activation switch through the pressure switch at the relay (with the activation switch in the on position, of course). Then work further down the line, and see if the 12v is showing at the connection to the heater element. If you have voltage at the element, then we can presume the element is burnt out, or otherwise somewhere in between you're not getting voltage.
On that note btw, I have a question, hopefully not to hijack. Does the pressure switch have to see at least some pressure to activate the heater? Because I was trying to test out my heater with an empty bottle, and (from the above mentioned procedure) I determined the pressure switch isn't closing the contact, thus my heater isn't working.