I'm gonna have a look round all the tracks and try and make a list of background cars. (I'll try to make pictures available ASAP). This is just what I think they are, so once I do get pics up, feel free to throw in you suggestions. NB: I will not include trucks and cranes etc that belong to track officials. I will include official vans however.
NOTE: THIS IS A WORK IN PROGRESS
2007 Mazda6 - High Speed Ring
1999 Cadillac Escalade - High Speed Ring, Cape Ring, Sarthe, Fuji
1986 Chevrolet Camaro - High Speed Ring
1976 Ford Fiesta - High Speed Ring
1997 Mitsubishi GTO - High Speed Ring, Sarthe, Fuji
1989 Toyota Chaser - High Speed Ring, Cape Ring, Sarthe
1977 Ford Granada (European version) - High Speed Ring
1994 Fiat Ducato Motorhome - Cape Ring
1995 Toyota Granvia - Trial Mountain
2000 Iveco Daily - Top Gear Test Track
2010 Chevrolet Spark - Cape Ring, Sarthe
1998 Nissan Cube - Cape Ring, Sarthe
1996 Mitsubishi Legnum - Cape Ring, Sarthe, Fuji
2000 Mazda Tribute - Cape Ring, Sarthe
1990 Suzuki Alto Works - Cape Ring, Sarthe
1995 Mercedes-Benz Sprinter - Cape Ring, Tsukuba, Sarthe
2008 Subaru Impreza WRX STi - Cape Ring
2007 Honda Civic Type R (US Version) - Cape Ring
2009 Subaru Legacy Sedan - Cape Ring, Sarthe, Fuji
1990 Volkswagen Transporter T4 - Autumn Ring
1995 Nissan Sentra - Tsukuba
2001 Acura CL - Tsukuba
2001 Audi A4 Avant - Tsukuba
1992 Honda Accord (European Version) - Laguna Seca