FOR MEGA MONEY DO THE FOLLOWING STEPS

STEP 1: Sell you small engine and buy a small tower
STEP 2: remove your small wheels and go to SELL PARTS
STEP 3: click on your small wheels so it will be able to sell but DO NOT SELL
STEP 4: go to your paint shop and at the very top of the right hand corner where you paint your tower move you mouse around slowly than when a hand pops up instead of the mouse arrow CLICK IT a glitchy looking SELL NOW sign appears
STEP 5: keep on clicking the SELL NOW button than go to buy parts and look at all the money you have and then buy an armour regenerator and do it with it because you get credits faster

PS. It will work without an tower at 1st you just need to know where the right top corner of the small tower will be at the paintshop