This is a known issue, and unfortunately so common on Berlingo. The alternative looks like that after you switch on the seat heating after about 5min, turn it off again (indicator light goes out), and turn it on again after a few moments. You can actually repeat this indefinitely until the temperature in the car is deliciously warm.
Otherwise, you can simply leave the seat heating switched on even after the first heat up on longer journeys. Depending on the environment, this switches on after 20-30min.
The seat heating is definitely a bad investment when ordering new, and also a technical joke!