The new franchise has replaced MG Rover and comes with a refurbished showroom to Nissan standards.
Service reception is included in the showroom with a workshop now situated to the rear of the premises.
The work took three months to complete.
Adding Nissan takes the group back to five major franchises, three at Wallasey – Nissan, Citroen and Mazda, and two at Heswall – Peugeot and Fiat.
Windsors, established in 1921, operates sales service and parts for each of the franchises held and also operates a bodyshop in Wallasey. The group employs a workforce of 90 across the two sites.