I expect the eBay ones will not be heated at that price, or heated in a Chinese Electronics manner.
The last time I took a heated door mirror to bits was on a Cavalier. Motorised non-retracting TV-screen sized things that cost £100 to replace, so were prime targets for thieves, myopic drivers and people walking home from the pub looking for something to kick.
The heated element was contained within the glass assembly, a replacement was the glass / element / plastic back sandwich which clipped into the motor housing with a couple of wires (iirc) for the heater.
One time I got a Halfords stick-on replacement which adhered over the top of the existing (broken) glass. Great idea except, the heating element now had double the thickness of glass to deal with (and a nice insulating sticky layer), and basically couldn’t cope.
So, based on my experiences of 1990s technology, I’d suggest you need to decide how important heated mirrors are to you before taking the eBay route.