The only thing I'd say about 3D printing using a thermoplastic is the heat on the car on sunny days. The plastic can warp and sag.
So it is a problem with some materials. PLA is going to be mostly useless as you can deform it using hot water from the kettle - anything >50°C and it doesn't really like being exposed to the elememts. There are places for it, but there are many materials that do much better, but at the cost of surface definition. PETG is probably a good choice for interior bits
To answer some of the questions about coloured parts as I think earlier my head was more aligned to customisation that outright replacement - it's doable, but possibly some kind of dye or paint would be a better option if yours isn't broken. You end up with tooling marks where the material has been laid. Aesthetically not the best solution. I'd think a resin might be much better in this case, but I've really limited knowledge of that in terms of durability.
The external/under bonnet stuff I think should be catered for with nylon. Edinburgh, I'd need to have a think on the fuel rail clip. Doable, but you have to consider orientation when printing so you can avoid support material.
Dave, loads of good ideas. The strut top caps I think could be replicated in TPU, just a case of if you could replicate the shape close enough. But I guess a cap would be a cap at the end of the day and help prevent rust.
The bonnet release handle I think I can do. I have planned to take mine off and see what I can do. There should be more than enough strength available. Just a case of getting the print orientation correct.
Some really good ideas. Thank you all