It's not impossible. Far from it - it's just very difficult. I have a team of several professionals at my disposal - specialising in reverse engineering, decompiling, hardware forensics etc - and they said it would be a ten minute jobs tops (to remove the reference to Bnk1.map, whether that works or not is a whole other story). But since they are professionals they won't do it cause it's technically game cracking.
So I can tell you what I know, and that is. It's impossible - for you at least.
You can go two ways about it - 1)Editing the exe or 2) Editing Bnk1.map.
1. Editing the .exe
Do not think of the exe as packed, but rather compiled. Every software is written in a language - C++, Java, Python or whatever. Once the program is compiled, it is no longer stored as this code, but rather something much more complex, called machine code or an assembly language. Which is pretty much the lowest level of programming there is. You can decompile TestDriveUnlimited.exe into this machine code and edit it, but knowing what it means takes extreme skill. And I don't mean "Oh, I'm good with computers and can make pwetty games, 3D models and edit the registry", I mean absolute hardcore knowledge about the interactions of software and hardware. Still, if you wanna give it a shot:
http://en.wikipedia.org/wiki/Machine_Code#Example
http://en.wikipedia.org/wiki/Assembly_language
2. Editing the bnk1.map.
Djey's spent two years on it. We still can't add cars. 'Nuff said.