You can also get the PST rebate form online. I have had hit and miss success when asking an Autopac agent for it - some don't even seem to know it exists.
The form is called the Manitoba Retail Sales Tax Act - Application for Refund Motor Vehicles. I just typed "manitoba retail sales tax act motor vehicles" into Google and it popped up as a PDF.
Hopefully this link will work as well:
https://www.mpi.mb.ca/en/PDFs/refundmotor.pdf