Newish to 3d modelling and I am pulling my hair out, please help

[aholmes28191]

Hey guys, Can someone be so kind and help/point me in the right direction. I have an FDM and resin printer (of which printed my first using FDM but I want to try printing this one in resin as it will be more a display piece , first one I purchased pre made now I want to do another shell but isn't available on any of the sites as an R/C shell. The shell in question is for a tesla model 3, I have managed to track down a 3d model but what I am looking for is the shell with window, front and rear light inserts bonnet removed as I want to print these in clear resin and add modifications.I have tried meshmixer, blender, windows 3d tool, watched hours of Youtube videos (nothing as specific to what I am looking for), to try and hollow this model and trim out the other parts, no matter what I try I cannot get the desired output. meshmixer wont let me cut through inner side of shell, blenders solidify just refuses to work for me and windows 3d builder does it but makes a mess internally and doesn't leave a smooth-ish finishI mean my theory is I previously purchased an STL pre made so surely there is a way to do this, but I am at a loss. I have added some files to show the mess I've made so far. Also just t If anyone can point me in the right direction I would be very grateful. thanks