Every radio has a unique code.....that's why it's there.
A "Default" code would be useless as it is there to stop people nicking the radio and selling it on....think about it.
Try taking it out and see if there's a lable on top (shouldn't be!......but most people, like me, use a felt tip pen to put the code on...no-ones gonna wanna steal it anyway!) or look in the original user manual.
BTW....what "is" the original.....they came with various units installed...mostly Blaupunkt (or Grundig), but different model numbers.
Radio, radio/cassette, radio/CD.....
Can't believe it's the 1st time in 18 years the battery has been disconnected!!!! (Must have had at least 3 new batteries in that time...maybe 5!)
BTW..they stopped making T4s in 2003....so yours must be "old stock", stored/converted, and eventually sold..

It could even be a 2000 model as far as you know.....check the details on the lable on the fuse box cover, or the back of the manual.
The VIN should also tell you date of manufacture.