Hy Abarthswed,
if you want to go back to Sweden sooner or later, I wouldn't think of registering the car in Germany.
If ,on the other hand, you want to stay for a long time, you should make sure that everything you modified on your car does not affect the driving or handling of the car in any negative manner.
For a german registration you will have to have all papers for the car, like a title (as for US-Cars), Swedish registration and if you have something similar to our Tüv, the last report. (for example France has something similar and they wanted that damn last report 
)
And to answer your question: The Tüv doesn't care about where the car is from. They are only interessted, weather the car does fit their regulations.
If you changed the exaust, it has to be within a fixed defined decibel limit. (I don't know the limit, but there should be somebody with some experiences!!!)
Since our cars are no Golf II or a Volvo 440 most people at the Tüv don't know how the original exaust did look like. Therefore, if your car does not produce too much (great 
) noise, you should pass even with a do it yourself modification.
In order to help you, if it will be necessary, you should write about the exact changes to the original in here, hoping, that anyone has something similar AND passed our technical inspection! Because than they have a peace of paper with the written acception. And that is what would help you a lot! For example the tires, any changes to the seats and so on...
If you have all these papers you need the so called "Unbedenklichkeitsbescheinigung" for your car. That's a paper that will proof that your car is not listed in Germany and is not on the stolen cars list.
You can get it at the KBA in Flensburg, that's where all german cars are listed at. But it is only worth for a month. You will have to get a new one after a month passed by without a successfull registration.
So this will be the last thing to get. Tell me if you need it, I can get it for you. I am living 5 minutes from the KBA and the procedure takes about a day, sometimes faster.
I hope I could help you a little bit. I don't want to shock you, but specially for modified cars it is a rocky road to go!
ChrisFL
und ich dachte hierzulande fahren schon genug kranke Penisverlängerungen herum.
die Wurzeln wären zumindest bei Fiat! 
Mein A112-Schraubergenosse Michi hat seinen 89er Uno mit einem Container bekannt gemacht.