I think you mean Social Security and Medicare taxes (not Supplemental Security Income which has nothing to do with this).
The employer should have grossed up value of the vehicle and used the additional amount to cover tax withholding, including Social Security and Medicare taxes. For example, if your federal tax withholding rate is 20 and your your state withholding rate is 5%, for the vehicle valued at $17,000 , the amount added to box 1 of your W-2 should have been $17,000 + $17,000 * (20% + 5% + 6.2% + 1.45%) = $22,550.50. $3,400 should have been included in box 2, $1,054 in box 4, $246.50 in box 6 and $850 in box 17.
Or perhaps the value of the truck was considered to be less than $17,000 and $17,000 is the grossed-up amount. In that case, the above example would have a vehicle value of $12,815.68. $17,000 would be added to box 1, $2,563.14 to box 2, $794.57 to box 4, $185.83 to box 6 and $640.78 to box 17.
Whatever amounts needed to be included in boxes 2, 4, 6 and 17, the employer was responsible for sending these to the appropriate taxing authority.
