Been there....done that. I was flying at night into Ocala, FL. The panel lights were poor, and i could not see the top of the DI so i maintained the reciprocal heading at the bottom of the DI which was illuminatted much clearer. I had been following this for an hour and began to make calls for a straight in approach. Didn't realise until finals that it wasn't 18 i was heading for but 36. A horrible feeling, but you really do learn from it.