I'm a Sales Exec for Audi, I get a piddly basic with a complexed commission structure that tbh I can't be bothered to explain at any legth.
I've been at Merc, VW and now Audi and it is generally much-of-a-muchness. You'll start slow... then you have the "beginner's rush"...then your figures dive... then you level out with a couple of quiet patches. Once you've been in the trade for a couple of years you start to build up a customer base and you start to get repeat business. Your managers should understand this and they shouldn't hold anything against you for fluctuation, just as long as customers keep giving you good feedback and you don't stick out from the team.
Now I know you are in America and this could change a lot of things in terms of stability, pay... all sorts. All I know is for me, always working for a main dealer I have a relatively stable job unless the s*** hits the fan or I screw up BIG time. Back street dealers are very aggressive, you end up knuckle fighting for the business and no, I would not recommend it. In terms of security you could look at the number of sales executives, depending on the size of the dealership;
3-4 sales execs in a dealer like Lexus and you'll have a big to-do list and plenty of sales opportunities, although you may get kicked for sales ratios (new customers v. test drives v. deals)
5-7 and things should be comfortable.
8+, you'll need some boxing gloves.
Perhaps ask one of the more honest looking employees of the company (receptionist, parking attendant, maybe even a young sales exec or service advisor) what employee turnover is like, managers will always inflate it.
I have never seen a service 'manager' move to sales before - maybe because it's just me, maybe because nobody would do it. Personally I can't see the advantage in switching, but the grass is always greener on the other side
