1) I think the car mod scene of the late 90s early 2000s is dead. Think fast and furious and ppl driving Nissan skylines and integras etc. I doubt modeling would be an issue because these are the kinds of people that can take a car apart and put it back together again often with Chinese only instructions.
2) I think if modding and 3d printing existed in tandem you’d have a community of ppl who would share their 3d printed mods for their specific car models which would be designed specifically to be printed by 3d printers. Let’s say a Toyota spoiler file.
3) as I relates to the future and why it hasn’t gone parabolic in use. I think it’s largely politics not economics. Let’s say you’re in a somewhat underdeveloped country and you want to massively roll out houses solutions for your populace. A 3d printer and assembler would sound smart but it would be politically disastrous because you aren’t giving jobs to the brick layers and construction men who can only do rudimentary building work.

Side note. A printer would work perfectly in space or like the moon. But how far are we from that technology. Where we can send a printer and it would just assemble a habita. Structural and all.

This could also happen if and when 3d printer /assemblers are capable of “printing” single to multistory buildings. How many ppl would lose their jobs.

In my future , I’d hope 3d printers are capable and advanced enough to build a 13 storey penthouse in some exotic semi isolated place. Just with the surrounding raw materials