Did it
Have had two Reliable's. They appear to last 1.87569 years each before death
When are you retiring?
Unplug it awhile. Just 30 seconds. When you plug it back in and engage the steam heater you shoukd hear or feel a click/thump/tink. If not, the relay is not sctivsting.
It's a little box with four wires. Two on each side, likely with blade connectors. Two are control voltage, maybe relatively smaller wires. The other side will have one wire going to the heater directly or through the fuse. The other wire will probably go strsight to the plug wires or to them through the fuse.
If you are not trained in electrical do not open the box with it plugged in. You want to go home in your car, not being driven home with electrical burns.
Unplugged!
Check the solidity of the four wires on the boxy thing is first step.
If you have a meter: (the ohms setting may say resistance or just have an Omega-looks like a pair of headphones)
check the heater ends on the ohms setting. It should read well less than 1000, likely around 20.
Check the relay control voltage input points on the ohms setting. . That should read very very low too. The control voltage just energizes an electromagnet. The coil is what you are testing here. Bad coil, no magnet, no pull to close the circuit.
That's about all the safe things to test.