A following driver saw a man confronted by his killer, a jury heard.
Wealthy businessman Ken Harvey, 44, was shot as he sat in his Mercedes at an isolated crossroads.
Mr Harvey, who ran a courier company, was shot five times by the pillion rider on a motorbike at Cripps Corner, near Battle, two years ago.
Police believe he was the victim of a contract killing linked to drug trafficking - the father of three had been due to go on trial accused of importing cocaine worth half a million pounds into the UK.
Raymond Taylor told Lewes Crown Court he was driving from his home in Broad Oak, near Hastings, to work at 6.45am on August 18, 2003, when he came to a halt behind the Mercedes at the junction.
A motorbike was stopped in front of the car. He saw the pillion passenger talking to the car driver and assumed he was asking directions.
Mr Taylor said: "He ran around the front of the Mercedes and jumped on the back of the bike and the bike went off quite fast. That is when I thought something funny was going on."
Mr Taylor revved his engine to indicate he was waiting to drive on. When there was no response he feared the driver had been the victim of a road rage attack and another motorist approached and said he believed there had been a shooting.
Mr Taylor said: "I looked in the car and saw a man slumped over. I could see a hole in the side of his head and blood on his chest."
Mr Harvey, who also lived in Broad Oak, died two days later in hospital.
Colin Meek, 35, of Sanderstone Road, Leyton, East London, and Jason Bradley, 37, of Claygate Crescent, Croydon, both deny murder.
The trial continues.
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