Well I think there can be two reasons for this.
Maybe there is a setting in the F1grapics.cfg file that can disable the cockpit visor. I dunno if this is the case, or if it is, what setting could do that. But you could try and replace your f1graphics.cfg file (by backing it up and deleting it, gp4 will generate a new one) and see if the problem persists. If replacing the f1graphics.cfg cures the problem, then you only need to work your way through the file and find the setting that needs to be changed. By comparing the two files, there shouldn't be all that many settings that you need to look at.
Another possibility is that the visor tex files in the circuit files - [circuit].wad - are empty. There are 13 files that you should check, and they can be extracted and viewed with an editor like CSC4 or Gp4master. Look for:
Visorlower.tex
Visorupper00.tex
Visorupper01.tex
Visorupper02.tex
Visorupper03.tex
Visorupper04.tex
Visorupper05.tex
Visorupper06.tex
Visorupper07.tex
Visorupper08.tex
Visorupper09.tex
Visorupper10.tex
Visorupper11.tex
If they are empty simply download some new ones, like Realistic Helmet Visor by Viper, or extract them from the circuit files on your cd, and insert them with Wadupdater.
Post Edited (06-09-03 15:52)
It's only after we've lost everything, that we are free to do anything.