Look at the list of most stolen cars. The most stolen cars are stolen for parts for the car was a hot seller 4-5 years before and thus still a common car on the road.
You do not want something more than 10 years old, as cars get over 10 years old they start to be withdrawn from the market and become rarer and rarer (and thus start to stand out). Newer cars are heavily advertised so they tend to stand out in people's minds.
The ideal car is a car whose model style changed within the last 1-3 years but was in production for 5-10 years. Most car makers change the style of their cars about every Five years (Through some models can last for Decades for example the 1973-1989 Chevy Pickup and other are changed within a year if they do not sell). Thus you want a car that is still common on the road but NOT being advertised on Television as a new car.
As to color you want a car that does not stand out, white, gray etc are good colors. Red have become more and more a common color since the introduction of Polyurethane Paints in the 1980s (Prior to that time period Red would fade to orange after a few years so NOT popular). Remember it is NOT the brightness of the Color but does it stand out? A red car among other red cars do not while a gray car among red cars will (And white vans and trucks are one of the most common color work vehicles today, remember the DC Sniper and the report of a "White Van"? The report was do to the fact white vans are Everywhere).
The last time I check the most popular car colors were White, Black and Red. Variations of these will also work.
Thus a white minivan of a style (Notice I said Style NOT model) no longer in production would be the ideal surveillance vehicle. I will stay away from large vans for people have been joking about Large White Vans and Police Surveillance since at least the 1970s, but a mini-van in a dark red? (a very common Color Mini-vans) might be ideal.
Some articles On Most stolen Cars with lists:
http://www.bankrate.com/brm/news/auto/car-guide-2004/most-stolen1.aspThis look like it for 2000 so a little old:
http://www.bankrate.com/brm/news/auto/car-guide/stolen.aspThis is for 2004
http://www.statefarm.com/insuranc/auto/nicbcars/grndthft.htm