I have no idea. It is about 4" in diameter, 12" tall, mounts high on the passenger firewall, and hoses go through the firewall and under the dash. Maybe its not a vacuum canister. Maybe something for the heater? Haven't torn into it yet to see where it goes. hmmm. I'll get a picture to share w/ you guys.
Yeah, the roof has been a landing pad for numerous square bales, bags of feed, a good place to take a nap while waiting for another load of grain from the combine.