I use a 10 foot 3-legged aluminum orchard ladder. With three legs you can kindof straddle one corner of the front - on both the driver's side and the passenger's side - because the back support leg can be spread out much further from the two main front legs than with a conventional ladder.
This works pretty good to reach each half of the front cabover area, one-half of it at time. Here's what an orchard ladder is:
http://www.gemplers.com/product/146886/Aluminum-Fruit-picking-Ladder?gclid=CJe1g8af370CFcqUfgodCVgAdw&sku=146886&CID=25SEPLA&ci_src=17588969&ci_sku=146886&ef_id=U0AbcgAAAK-YSmtb20140414052947s