Even slight changes in packaging to adapt the panels to a structural material might increase cost. But doing custom shapes to fit the panels of a car? Getting just the right shape? That's an entirely different manufacturing process nothing like what currently gets produced.
The benefit on cars is also pretty small, at least whenever I've bothered to calculate it. Cars need a massive amount of energy compared to a house, and have far less surface area.
Sono Motors tried it and went under..
In theory, it sounds great and even obvious, but once you get into the details - not so much..
We could build seasonal overcapacity today, and it would be nearly economically competitive with existing fossil sources.
It's hard to internalize just how cheap solar panels have become. Keeping the same inverter and grid connection, and doubling the panel capacity, nowhere near doubles the cost of the install.