I had a bricked GPU and it didn't display anything in PC or Mac. And it was not possible to use another videocard either. If it was installed the PC would not boot at all.
But, then I managed to flash it out of computer all together. And its also possible to buy a new ROM-chip and flash it with a CH341A Mini Programmer and then solder it into place of the original. I was ready to do that but then I managed to flash the original chip in situ with an adapter that came with the programmer.
This was not a Radeon X1900 but I guess the same approach could work with them too (?).