I'd Better Take My Car in for Service

There is no past form. You can't use it in the past. It's only used to talk about things you should do in the present or the future. The form is fixed, had better and followed by a verb in the infinitive but without two. For example: I'd better take my car in for service because I wouldn't like to have an accident - would I? And notice there that Reza was giving himself some strong advice with I'd better leave now so that I miss the rush hour traffic.