You were leaving because you were tired of the headaches but you're going from the frying pan to the fire? Your headaches will increase exponentially as management. I'd take my best year of earnings there and double or triple it as a starting point but keep in mind that sometimes the money just ain't worth it. If you've had it with them then more $$$$ only extends the agony. If you plan to stay there make it well worth your while.
Seriously??
OP doesn't want to give the dealer a reason for the OP to leave, right? If I had an employee asking for that amount (even during the inevitable wage haggling) I'd look at him or her & want to know why they think they're worth that price and be able to back it up.
OP mentioned 2-3 cars per day which would tell me that they aren't THAT busy.
OP also stated that he's currently working 50+ hours per week. Tripling or even doubling past earnings as a starting point is going to allow the dealer to let the OP walk out the door without a second thought. Is the OP bringing work to the dealer or is the dealer selling an in-house service that the OP has been hired to provide? Toyota, Lexus and Honda owners are not the "high end detail" type of owners anyway.
OP also stated that he's currently working 50+ hours per week. OP is most likely making some overtime here, right? The overtime disappears (in most circumstances) when an employee migrates to salary rate of pay. In the OP's case; I would look for a percentage of a total value for each detail that is up sold to the customer as an additional incentive for the OP. Maybe base the salary rate (higher rate contingent on level of volume increases or level of customer satisfaction) with monthly bonuses as an option.
The dealer obviously recognizes the fact that they need this guy to stay with the detailing department. That said, they're also going to look at the value he has brought to the dealership(s) in regards to revenue. Salaried employees can expect at the least a 50 hour work week without overtime and with more headaches (as stated above).
Long & short of it?
OP needs to be able to justify the wage he asks for rather than picking a starting point. The upper management will respect that and the OP will come across as being a competent & integral person within their organization.
OP needs to have a game plan ready & work with the fact that the dealer asks him to stay. This gives the OP the reigns, as it were.
OP:
Sell yourself and your experience in a way that your employer sees nothing but reasons to retain you, but without being overly confident about it. :xyxthumbs: