The attacker grabbed her after she had parked her vehicle in Castleview, Dundonald, at around 2.35am.
Local residents came to help after hearing her screams and the man ran off.
His 32-year-old victim suffered minor cuts and bruises during the attack.
Police appealed for information about the incident in Ayrshire.
Detective Inspector Alan Somerville, of Kilmarnock CID, said: "This was very distressing and upsetting for the lady who had just popped out on an errand.
"Thankfully her screams alerted nearby residents who came to her aid and the man ran off.
"I would like to speak to anyone with information about the attack, especially any of the residents who helped the lady but who were not at the scene once police arrived."
The suspect, who smelled of alcohol, is described as 5ft 8in to 5ft 10in tall, of chubby build with short hair.