TrinityCore/TrinityCore — pull request #7598 by cyberbrest (2012-08-31 19:41:57+00:00)
Title: [Core/Spell] SpellImplicitTargetInfo::TARGET_UNIT_TARGET_PASSENGER use TARGET_REFERENCE_TYPE_CASTER

...

and add rule for handling it it's just like for TARGET_UNIT_PASSENGER_0-TARGET_UNIT_PASSENGER_7.
i think that it should find any unit in seat.

Research based on spell 66298:
ClientToServer: CMSG_PET_CAST_SPELL (0x6337) Length: 18 Time: 08/06/2012 19:18:48.386 Number: 6929
GUID: Full: 0xF15088180005C566 Type: Vehicle Entry: 34840 Low: 378214
Cast Count: 26 (0x1A)
Spell ID: 66298 (0x102FA)
Cast Flags: None (0) (0x0000)
Target Flags: Self (0) (0x0000)

ServerToClient: SMSG_SPELL_START (0x6415) Length: 37 Time: 08/06/2012 19:18:48.635 Number: 6933
Caster GUID: Full: 0xF15088180005C566 Type: Vehicle Entry: 34840 Low: 378214
Caster Unit GUID: Full: 0xF15088180005C566 Type: Vehicle Entry: 34840 Low: 378214
Cast Count: 26 (0x1A)
Spell ID: 66298 (0x102FA)
Cast Flags: HasTrajectory, Unknown16 (262146) (0x40002)
Time: 0 (0x00000000)
Time2: 0 (0x00000000)
Target Flags: Self (0) (0x0000)

ServerToClient: SMSG_SPELL_GO (0x6E16) Length: 48 Time: 08/06/2012 19:18:48.635 Number: 6934
Caster GUID: Full: 0xF15088180005C566 Type: Vehicle Entry: 34840 Low: 378214
Caster Unit GUID: Full: 0xF15088180005C566 Type: Vehicle Entry: 34840 Low: 378214
Cast Count: 26 (0x1A)
Spell ID: 66298 (0x102FA)
Cast Flags: Unknown7, Unknown16 (262400) (0x40100)
Time: 0 (0x00000000)
Time2: - (0xFD602118)
Hit Count: 1 (0x01)
[0] Hit GUID: -Type: 8 Name: -
Miss Count: 0 (0x00)
Target Flags: Unit (2) (0x0002)
Target GUID: 0x0