Barrington Clarke told jurors two shots rang out, fatally injuring Dwaine Haughton as he sat in the passenger seat of his Peugeot 207.
He recalled the tragic moment he realised the shots had shattered the car windows and seriously wounded his 24-year-old cousin sitting by his side.
Giving evidence from behind screens, he said: "Two shots [were fired]. All the car windows went through and Dwaine basically fell onto my lap.
"I could see an open wound on his head. I reversed the car and sped off towards the hospital."
Prosecutors allege at least one of the four occupants of a stolen grey Audi A7 on false plates got out of the car, firing several shots into the Peugeot at about 9.10pm.
Mr Clarke said he had parked in front of someone else's garage in Park Village's Valley Road when he soon "became aware" of a gunmetal Audi A7 approaching.
Jurors heard Mr Clark explain the Audi pulled up about 4 metres away from the passenger side of his car before shots were fired, hitting Mr Haughton in the head.
He then rushed to New Cross Hospital in his car, calling on two police officers for help when he arrived on July 20 last year.
Mr Clark added: "The police officers went to get a doctor straight away. The doctors came out of the hospital and tried to treat Dwaine inside the car first."
The witness went on to claim he caught a glimpse of the driver of the blacked-out Audi, who had a "smirk" on his face during the shooting.
He could clearly see the driver's face as the window of the Audi was down, but could not recall seeing anyone else in the vehicle or grasping gun.
Jurors heard how Mr Clarke was allegedly shown a picture at hospital of defendant Kurrum Guild, who he claimed was driving the car.
But he failed to tell police he believed Guild was connected to the shooting until July this year, Wolverhampton Crown Court heard on Tuesday.
Mr Clarke added: "I recognised him to be the driver of the car. It was still fresh in my mind so I was pretty certain."
Mr Kevin Hegarty, defending Guild, put it to the witness that he did not see the driver of the Audi.
Clarke replied: "I did see the driver. I was in shock, I panicked. I did not want to get into any trouble for what had happened to Dwaine.
"I had just lost somebody who I was with every single day."
Mr Hegarty continued: "You simply have got it wrong haven't you?"
Mr Clarke replied: "No I haven't. That's what happened. You weren't there that night."
Guild, of Oakthorpe Gardens, Tividale, 26-year-old Dwain Smith, of Chainmakers Close, Bilston; Seamus Williams, 23, of Okemont Drive, Wednesbury, and Montell Gray, of Springfield Road, Wednesbury, deny murder, possession of a firearm and ammunition to endanger life and handling a stolen car.
Smith, Williams and Guild also deny arson.
The trial continues.