The attack was an intimidation attempt by a paramilitary group, says the garage owner.
Peter Johnston, who won in an election with a 22% turnout, is the son-in-law of DUP MP Paul Girvan.
The victim was hit with a fire extinguisher and suffered a fractured skull in Carrickfergus attack.
Police carried out a controlled explosion after the object was declared a viable device.
The victim was treated in hospital after the incident in Carrickfergus.