Just get a complete known working unit from another car,maybe redo the headlining at same time.
It's a very straightforward job although a bit long winded maybe.
Bear in mind there are 2 types of cassette and each has a different sliding panel.You can interchange but you need both parts,cassette and panel.
Best bet would be to get to the one youve got already to see what type you have.
What area are you in ?.