I'm not sure if this place has been mentioned before but the Poonsap Market in Pathum Thani is exclusively for car and motorcycle accessories, and at very cheap prices. There are a good number of units there selling everything you could want in the way of accessories for cars and pickups, as well as parts for motorcycles. Prices are often negotiable and usually include fitting.

The market has a large sign in English and is located about 100 meters from St Carlos Hospital in Rangsit-Pathumthani Road.

I've been there quite by accident so really no had any idea how to find it again. I remember though because the name St. Carlos hospital jumped out at me in Thailand so likely it's the right place. Lots of accessories not quite like China town but still quite a wide selection.

St Carlos is in Tiwanan Road right at the junction of Tiwanon Road and Rangsit-Pathum Thani Road. The market (coming from the hospital) is about 100 meters on the right in Rangsit-Pathumthani Road. Its hard to miss as there is usually a big large number of parked cars outside. The market entrance has a big elevated sign in English, but parking inside is difficult due to the alleyways. Best time to go is on a working day when its not as busy as weekends.