You had to hard-code the makes for the LIKE strings. Why not just hard code them in a literal for the make column?
SELECT
tDealerships.CapShortName,
SUM(tObjective.CommitObj) AS CommitObj,
SUM(tObjective.ActualMTD) AS MTD,
SUM(tObjective.DelToday) AS DelT,
SUM(tObjective.ActualMTD)/Sum(tObjective.CommitObj) AS CommitUnit,
SUM(tObjective.CommitGrossObj) AS CommitGrossObj,
SUM(tObjective.GrossActual) AS GrossActual,
SUM(tObjective.GrossActual)/Sum(tObjective.CommitGrossObj) as CommitGross,
Sum(tObjective.ActualMTD)/Sum(tObjective.GrossActual) AS MTDPRU
------
'GMC/Buick' As Make
------
FROM tObjective, tMake, tDealerships
WHERE tObjective.DealershipID = 10
AND NewUsed = 'New'
AND tObjective.MakeID = tMake.MakeID
AND tObjective.DealershipID = tDealerships.DealershipID
AND (tMake.Make LIKE '%BUICK%'
OR tMake.Make LIKE '%GMC%')
GROUP BY tDealerships.CapShortName