Initially you ask for the sales managers. (new managers have a seperate budget than the used one).
I never offered free trials and they did not expect them, it looks desperate to me.
Tell them you are putting a unit in (city).. and need to book your days accordingly to secure them a slotted interval. Either weekly or bi-weekly. That lets them know they will be looked after by a pro. Enter into a verbal contract with them that that time is theirs and you need two days cancellation notice from them otherwise they are paying anyways.
If you do a good job and are reliable, up your price after six months, if they walk, they will be dissapointed at the new providers lack of not knowing the routine you and they have developed and will eventually return.
It will get to the point where they have given you free reign on the cleanliness of their cars and you will be stylin'.
Hope that helps..
Watch those rocker panels!