I'll try and get some screenshots to update this later. I'll write it up quickly for now and try and make it better later
*** You will need at at least one upgrade on a vehicle (more makes it easier) to use this method ***
First - make sure you've set your temp folder for GameGuardian!
Start CSR, go to the GameGuardian and choose the config options (left tab). Go to "Set Search Range" and select "More Regions"
Go to your Upgrades - pick an item (Intake is cheapest) and search for it's current "Stage".
Fit a different Stage (up or downgrade it) and search again.
Repeat changing fitted stage and searching until you get as few matches as you can. (Mine was 0x60C56424 this time. Has always been in the 0x6 area)
(You may notice in my screenshot below I'm selecting the "Intake" after searching on Engine - it was to try and make sure I didn't accidentally find the number in the text on screen, not a mistake!)
Go into GameGuardian and go to the 4th tab (the 4 horizontal lines) and in the "Address" section put in the location you found above.
This is where it gets a little trickier!
If you scroll up a little there should be a location with the value "0x00000007" - that is the start of your car.
Going down from there the next 14 lines are your cars upgrades! All entries should be 0-5 (more doesn't make any extra effect!)
Engine Stage Purchased
Engine Stage Fitted
Turbo Purchased
Turbo Fitted
Intake Purchased
Intake Fitted
Nitrous Purchased
Nitrous Fittted
Body Purchased
Body Fitted
Tires Purchased
Tires Fitted
Gearbox Purchased
Gearbox Fitted
The newer cars appear to be further "up" the memory. I was able buy a new car, quit out of the game and set my time ahead (setting the clock on your device ahead will cut out the delivery time!) and then search for my old car in memory. Then scroll up until I found the 0x00000007 and a load of 0x00000000 and change them all to 5 - and an instantly maxed out car.
** Edit ** - Added some screenshots (do they help?)