I left myself the option of trying a shark fin antenna over the same cable entry hole which would look neater if I painted it to match my trunk lid, but performance wise, a long straight dual band whip is going to be better than a short stub antenna and pre-amp. Unlike you, I have a cabriolet with a folding roof and glass fiber trunk lid. Therefore my r.f design included creating a ground plane on the trunk lid which involved more work.
Most DIY car antenna systems will work to some extent near DAB transmitters, but it's only out on the road as you move from one local DAB station coverage to another or get into areas screened by tall buildings that you can really tell their effectiveness. I think that's how Ebay sellers get away with selling most of the Chinese DIY junk which may seem to 'just work'. IMHO the antenna systems designed by OE manufacturers for new cars are going to be more credible.
The DAB-Z signal strength bars are quite useful to watch as you drive around. But I still haven't found out how to mute the radio or kill the app and still leave the Navi or an MP3 tune playing? Germany has a lot of DAB stations and German car makers should know best what they are doing if you look at newish DAB car models post 2018.
Putting the dongle in the trunk or even inside the head lining close to the antenna makes sense to reduces cable losses, but after a preamp either integral or mounted close to the antenna cable, losses don't matter so much because the biggest effect on noise figure will be losses at the first pre-amplifier input. Post antenna pre-amp and my active diplexer splitting stronger FM/AM from weaker DAB signals, I ran 2 lengths of RG174 - one to the head unit for F.M and the other to the glove box where my USB DAB+ dongle is plugged in. I removed every FAKRA connector and used plated SMA throughout.
Most Chinese head units have a selectable software option to produce 12v pre-amp power when turned on. From measurements, I found their d.c output was poorly regulated for loads above 50mA and was probably a simple source resistor to protect against shorts. My pre-amp and active diplexer draws about 100mA so I took an 'Always on' fused voltage source and switched it from the radio antenna d.c with a mini 12V relay.
All is not as simple as it seems.