OK, here we go again.

Install the 2004 GPL Demo from http://www.bhmotorsports.com/GPL/downloads/3239

Copy the cars folder to a new location.

Uninstall the 2004 GPL Demo.

Install the full version of GPL.  Take your copy of the demo's "cars" folder and replace the one installed by the full version.

Download the All-in-One patch from http://trackdb.d2g.com/all-in-one/GPL%20v1.2.0.2%20(US)%20all-in-one%20patch.zip

Unfortunately, it ISN'T an all-in-one patch.  So you have other steps to take:

Download CarSound.exe from http://www.california.com/~keeper/CarSound.exe -- save it to the "gpl" folder. Open a DOS window ("Command Prompt" in WinXP), move to your "gpl" folder, and run the CarSound.exe.
Make sure it worked successfully.  The new size of gpl.exe should be 1,820,160 bytes.

Download car sounds. My set is at http://www.california.com/~keeper/vroomsv5.zip -- drop these wavs into your "sound" folder.

Download the high-bandwidth patch from http://www.california.com/~keeper/pics/bwpatch%5b1%5d.zip -- put the .exe into your "gpl" folder and run it.

You should now have a fully working modern version of GPL (except for the tracks, which will look really crappy).  Download track updates!  The CRO site has updated versions of the tracks (some are a bit out of date at this point, but they're still literally years ahead of the ones from the GPL CD).

Now you can download the '65 mod with iGOR and so on by following the previous instructions.  What a pain!