A novel characterization method of OTA test setups for wireless communication systems on vehicles in the random line-of-sight (Random-LOS) environment is proposed. The measurement setup assumes a compact range and a test zone where the antenna under test (AUT) on the vehicle would be located. An ideal receiver is assumed for the reference measurement, which allows to perform a system analysis through evaluating the Probability of Detection (PoD) as the system figure-of-merit. The proposed method is aimed as an aid for test equipment designers to design OTA compact ranges, compare their performances, and define an ideal numerical reference. The requirements for OTA measurement ranges are different from those for conventional anechoic compact ranges. A compact cylindrical reflector system with an antenna array line feed is characterized using the proposed method, from 1.6-2.7GHz, for two orthogonal polarizations, various AUT heights and reflector tilting angles, with and without ground plane in a test zone which is 2m wide in diameter.