The difference between the present perfect and past perfect tenses is that the present perfect tense relates to something "past to the present" while the past perfect tense relates to something "past to the past". E.g. the ownership of the car has been going on until now (present perfect):
I have had a red car for a few months, but I sold it today.
The ownership of the car went on until yesterday (past perfect):
Yesterday I sold my red car, because I had had it for several months.
So the past perfect had had should only be used when you're describing a certain point of time in the past and the events before that certain point of time.
About your use of had had: if you went out every weekend when you had the car, the other had is one too many:
When I had a red car, I went out every weekend.
This is simple past tense, describing the past.
However, if you waited for two months after getting the car before you went out every weekend, then:
When I had had a red car for two months, I went out every weekend.
This is past perfect tense describing the two months leading up to the point in time in the past when you went out every weekend.