Short circuit in kitchen Studebaker

I just bought a 50-year-old Kenmore kitchen rotisserie that seem to be in mint condition. It works fine except it has the annoying habit of giving you a shock if you're not wearing shoes. Today I removed the non-polarized two prong plug and put in grounded three prong plug attached to the chassis. Trouble is it now trips the GFCI in the kitchen. Could not visually find a short. Any notion how to deal with this? Thanks!



This is what it looks like:

Vintage 1960 039 s Kenmore Oven Baker Automatic Rotisserie Broiler Grill Mid Century | eBay

Short circuit in kitchen Studebaker