1. As regards to your car, I think you should clean it.
2. With regards to your car, I think you should clean it.
3. In regards to your car, I think you should clean it.
There may be a difference between British and American English on this one, but I don't use the plural noun "regards" to mean anything other than good wishes.
I would use:
Regarding your car,
In/with regard to your car,
As regards your car (regards is a verb)
From the American Heritage Dictionary:
USAGE NOTE Regard is traditionally used in the singular in the phrase in regard (not in regards) to. Regarding and as regards are also standard in the sense “with reference to.” In the same sense with respect to is acceptable, but respecting is not.•Respects is sometimes considered preferable to regards in the sense of “particulars”: In some respects (not regards) the books are alike.