Dometic Waeco CoolPro TC-21FL (20L)

Put an ammeter on it and measure the current in the 10 amp range. Might have to go to an auto electrician to do this as a hobby type ammeter will probably deliver an 'out of range error' if it's working properly.
If, however, your $10 multimeter indicates a current draw is minimal, like 600ma or something in the range of what a larger brushless computer fan would draw, then the Peltier device won't be working. If that's the case it will be a 'pull it apart' scenario to see if perhaps a wire has fused a connection.

Under the unit it say's 4 amps @ 12 volts, that is provided you put cold items in to start with,these units only MAINTAINS items at the temp they were when they were put in to the unit,hope this helps.