The early control units had no diagnostics. Only the system could be tested. The next versions which you might have (you didn't state year) have impulse diagnostics and if the code ten is there then a new module is the answer.
The box for cars through 1987 004 820 11 10 should list for 1070. The unit for cars 1988 on use 000 820 68 26 at 1380. They aren't exactly easy to get to so 1500 could be a good price if it included labor.
Used parts are an alternative. Both devices selftest, so properly installed and system tested, used parts can save a bunch of money.
Bosch Master, ASE Master, L1
33 years MB technician