The whole Mr Kiper caper is explained perfectly well in AS book (pages 137 - 147).
The person who owned the car was called DR, Mr Kiper was the person who was the registered owner of the car, an elderly relative of DR. DR was not Belgian, he was an Israeli living in Antwerp, 33 years old and a diamond dealer.
The metallic blue two-litre BMW 518 was reported missing on 20 September 1986 in Knokke-Heist in Belgium. However, an english businessman called RW had noticed the abandoned car in Queens Grove, St Johns Wood in August 1986. Over the next few weeks RW became obsessed with the car, and eventually began to make enquires. In December, RW found out that the car was registered to a Mr Kiper in Belgium, remembered Suzy's case and the link to a Mr Kipper and a BMW possibly being involved, and contacted the police. After a few weeks of cutting through red tape, the police managed to interview DR in Belgium. He denied ever meeting Suzy or indeed knowing who she was.
However, he could not explain how his car came to be in London, why it was there in August but not reported missing until September and would not say how often he visited London. Also, he would not explain why the car was registered in Mr Kiper's name.
There were two main reasons however why DR was not considered a suspect.
1. DR's BMW was not the same model that RT had reported seeing in Shorrolds Road. RT had reported seeing a car that was similar to a 518 but was at least 10 years old, wheras DR's BMW was a 1980's model.
2. DR was able to produce a receipt from an Antwerp garage that showed his car was being serviced on Monday 28 July 1986 - the day Suzy disappeared. The police checked its authenticity with the garage's computer records and they confirmed the date.
Despite some mystery over the car, it doesn't look as if Suzy was involved with DR or any diamond heist.