While in the game, push [Enter] + [Shift] + [Alt] + [E],
then type any of the codes below:

4x4 - Big Tires
5x5 - Big Truck
99x99 - No Game Over

Hidden money:
On Farm Road level, go under the train trestle in the middle of the pond to find
$1500. Note: This works only one time.

On the Restricted Area track, go straight when you pass through gate 5. After a little driving you will reach an aircraft hanger with a jet inside. In the corner of the
hanger is a chest of cash.

On the Laguna Del Ray track, off on the right is a small island just offshore. On top the island is a chest of cash.

Between gate 6 and 7 on the Black Gold track is a fenced-in area that contains another box of cash. There is an oil truck nearby it.

When starting on the Treasure Bay track, go over to the beached tanker boat to the right, and drive around on the water side.
You can find a chest in the side hull, with $666.

On the Truck Stop 101 track, go to the "hidden" highway and drive alongside the right side of the road. You will eventually find a chest of cash.

Edit Cash:
First go into the game and play a bit to earn a little extra cash, at least $10,000.00 or so. Now sell your truck. Your should have somewhere near $40,000.00.

Look in your directory for the following file:
C:\programs\Terminal Reality\4x4 Evolution\system\metal.ini

..open the (metal.ini) file using notepad listed in your accessories folder in Windows. Now save the file in a temporary folder or on your desktop. Go back into the game
and buy a vehicle and some of the parts you want.
When your cash is low, do the following:

Exit the game and go back into your temporary folder and open the file named metal.ini and look for a line that says:
Cash=(followed by a series of numbers and letters)

I think the string is specific to the your player profile.
Select the line and copy it. (now this is important only select the line that says Cash=(and the string of characters following the equal sign) not the entire file). Now open the metal.ini file in your:

C:\programs\Terminal Reality\4x4 Evolution\system\metal.ini

and look for a line that says:
Cash=(followed by a series of numbers and letters)

Select that line and paste over it the line you copied from the previous file.

Now save the file and exit notepad. Go back into the game and like magic you just padded your wallet again. If you want more cash, like say enough to buy one of race trucks, just sell your truck again and repeat all of the steps above.
Remember every time you save the current:

C:\programs\Terminal Reality\4x4 Evolution\system\metal.ini

...your increasing the amount of cash you have. You can just keep doubling the amount of money you have by buying several trucks, pasting a new line in and then selling the trucks you bought. Saving that current file into your temporary folder and then buy more trucks and parts and on and on and on until you have the amount of cash you want.