A 30.06 sign wouldn't be valid for a parking lot AND inside your vehicle. 30.06 only applies if you are carrying under the authority of a license to carry. You don't need a license to carry concealed inside a vehicle under your control.
Sec. 30.06. TRESPASS BY LICENSE HOLDER WITH A CONCEALED HANDGUN. (a) A license holder commits an offense if the license holder:
(1) carries a concealed handgun under the authority of Subchapter H, Chapter 411, Government Code, on property of another without effective consent; and
(2) received notice that entry on the property by a license holder with a concealed handgun was forbidden.
Sec. 46.02. UNLAWFUL CARRYING WEAPONS. (a) A person commits an offense if the person intentionally, knowingly, or recklessly carries on or about his or her person a handgun, illegal knife, or club if the person is not:
(1) on the person's own premises or premises under the person's control; or
(2) inside of or directly en route to a motor vehicle or watercraft that is owned by the person or under the person's control.