my Seat from Harley Davidson is brocken

I am looking for someone near Bangkok, how can weld the HDPE plastic

HDPE you can not repair with glue, only welding

I had a similar issue in BKK and the easiest way to solve it was to buy a cheap plastic welder and DIY.  Cost peanuts, and they're available on Lazada.  Much easier than looking for someone, no Thai language required, and next time I needed it done, I already had the tool.

 

Basically similar to a soldering iron, with a flat tip.  They also sell cheap filler rod, but it doesn't look like you need any.  (In fact, if you have a soldering iron around, I wouldn't even bother ordering the special tool).

 

Edit:  OP may want to specify where in BKK, because shops that can do it are all over town.  No need to trek 20 km.  Personally, I'd use the zillions of shops close to Khlong Thom market in Chinatown, but that's because I'm there just about every day anyway.  That's where I stay on my monthly visits.

 

Drill a small hole on both ends of the crack and be done with it.