Hi! Have an issue with my neckthru 4 string warwick thumb from the 90's. The truss rod is difficult to adjust. Is it possible to have a new truss rod installed on this? Any recommended repairman in Norway or Europe?

Sveinis

I'd suggest contacting Warwick to see if they would take a look at it. If not perhaps they could suggest someone local to you.

Older Warwicks had an alluminium truss rod to keep the balance good...
But these rods aren't very tough. You should replace it with the steel version. A bit heavier but much better. Yes, the truss rod on your bass can be replaced without removing the fingerboard. But I'd suggest to have it done by a repair man.
Contact Warwick to see if they can send you the truss-rod. Or, if they don't help you, drop me a line and I'll try to help (I have a line to a shop that carries Warwick replacement parts). :)