Personally, I'd tell the new employer you feel you were deceived, are unhappy there and don't want to work in an us-vs-them environment between employees and mgmt. Tell them you aren't trying to negotiate a better rate (you were happy with the pay 5 weeks ago), but need to figure out if this fit can be made to work. Ask them how the situation might be handled or whether its best to just part ways now before anyone gets bitter.
That puts the onus on them to decide whether to improve the situation or call it a day. If they decide they don't want to modify things to make you happier they will very likely say let's just nip this in the bud today (rather than having an unhappy camper poisoning the other employees for weeks). Worst case they ask you to stick it out 2 weeks.
That said, it sounds like you went about it differently than I would and have already decided and committed to jump back to your old job. (Not sure I'd go back knowing that it took leaving to get them to pay me the market rate, but that's your call. Also doubt I'd take an employee back so fast...at a higher pay...because that's like telling all the other guys "hey just quit and we'll finally offer you a raise that would make you come back."..but that's another story.) Since you've already made that decision (and commitment?) don't screw around with the new (soon to be old) employer. Just tell them you feel deceived, are unhappy and are headed back to your old job (don't mention the higher pay at old place). Ask them if they prefer you work 2 weeks or just go now.
Either way, giving the employer straight scoop and the chance to decide will go a long way toward avoiding screwing yourself if one of their management moves to your old/new dealership down the road (and you are working for them again).
My 2 cents.