The the only way I’ve managed to get a discount with main dealers is to view the car, have a drive, sit down to seal the deal and when the haggling is limited to a set of floor mats/tank of fuel, get cold feet and say we are going off to think about it. This was after getting a bit stubborn when they wanted to charge us full RRP for a missing parcel shelf.
We said we’d ring before they closed 30 minutes later if we wanted to put a deposit down, but sod that, we got in the old car and drove off. Deliberately didn’t ring them before closing time, 10 minutes later the salesman was ringing us and suddenly the price had some movement in it, and they’d found a parcel shelf (i.e when we picked the car up, we waited whilst they nicked a shelf out of another car on the forecourt)
Still had to remind them about the fuel when we picked it up.