I think the lamp failure sensor will read an imbalance of power between lamps as a 'failure' so if your headlight pair, running lamp pair, stop lamp/tail lamp pair aren't 'balanced' they can cause a false failure indication.

We had an intermittent failure indication in our '96 E300 Diesel. It turned out the left rear lamp housing had been damaged by the DPO (Dreaded Previous Owner) and the contacts were damaged and would fail occasionally. You might have a look at your connections, a slightly loose connection could be doing it.

BCingU, Jim