My definition of a loaded weapon is any gun that has a magazine with bullets loaded into the magazine well. This means, if you have the magazine in but no bullet chambered it is still loaded. This is the common sense definition and what I think most LEO's would consider to be the case.
Here is the New Mexico law on carrying in your car:
30-7-2. Unlawful Carrying of A Deadly Weapon.
A. Unlawful carrying of a deadly weapon consists of carrying a concealed loaded firearm or any other type of
deadly weapon anywhere, except in the following cases:
(1) in the person's residence or on real property belonging to him as owner, lessee, tenant or licensee;
(2) in a private automobile or other private means of conveyance, for lawful protection of the person's or another's
person or property;
B. Nothing in this section shall be construed to prevent the carrying of any unloaded firearm.
I found this here.
http://www.handgunlaw.us/documents/USRVCarCarry.pdf
Here is a synopsis of NM law:
http://www.handgunlaw.us/states/newmexico.pdf