Jeep Fieldmodifications
Surge Tanks:
These tanks were added to achieve better cooling. They worked as a recevoir: steam would flow into the tank instead of escape and would condens there. The resulting liquid would flow back into the radiator. The tank itself was bolted onto the grill, and would thus be cooled by the wind. A special large version of the tank excisited for use in desert conditions. The normal "small" version was used in Europe, but was not as widely spread as for instance the wire- cutter. For pics click here.
These items were sometimes added to jeeps. Most were added/made available as fieldmodifications(kits). All are of WWII vintage.
Wirecutter:
An item very widely used in both ETO and PTO was the wirecutter. This was a T-shaped profile made of steel bolted or welded vertically onto the frontbumper in the middle. The top 5 inches were bowed foreward 45 degrees and mostly had some sharp edges cut in them there in order to cut wires. The top edge had approx. the same height as the top of the windowframe. This kit was introduced Italy because the German army had a nasty habit of stretching wires across roads between trees at neck-height. Sometimes seen on other types of Allied vehicles as well. (
see pic)
Winch:
At the end of 1944 a capstan winch was introduced to be used on jeeps, after 2 years of experimenting. This was a fieldkit, so it could have been fitted to any jeep. These kits are quite scarce and a price of US$2500 is not unusual for a complete NOS one. However, these kits were also made post WWII but these differed from WWII-kits in that they had the top lid of the capstan bolted down with 3 bolts instead of the WWII ones who had 4 bolts.
See pics.
Gun Mounts:
The following info comes from TM 9-224 (dated 04/43) and change 1 (dated 10/45). Info courtesy of Jeff Q. from Arizona.

Mounts:
M31: bolts to the floor via frame crossmember support  plate and 3 support legs. Designed for the D38579 pintle. Drawings to make one yourself can be requested at Jeepdraw (free).

M31C: same as above only it uses the E10014 Combination Cradle. Circa 1945 and later.

M48: bolts to the dash and renders your glovecompartment and windshield useless! Designed for the D38579 pintle.

Pintles/Cradles:
D38579: simple "fork" used with the .50M2, .30M1919 and BAR MG's. Requires a "trail arm" to keep the gun straight.

E10014: combination of the .30/.50 cradle featuring a detachable ammunition tray with a big "tension" foot. Similar to the cradle used on the M49 ring mount. Applied to the M31 pedestal mount per MWO ORD A55-W10, dated march 15, 1945 creating the M31C mount. Circa 1945 and later.

*** "The M25 mount is used only on the half-track personnel carrier M3" (quoting TM 9-224). It is very likely they found their way to jeeps as well.

*** Any short-pintle cradle will work in these mounts (.50 equilibrator cradle, .50 AA cradle, .30 M1917 cradle etc.) and appear to have been used per photos.
Equipment Carrier Rack:
Some jeeps were fitted with an extra equipment carrier rack, mainly in the ETO. These were large racks made from metal, bolted to the back of the jeep. Mostly they had spare-outs for the spare tire and the jerrycan.The rack, although ugly in appreance, did it's job well and allowed crews to carry quite an amount of extra load, such as field equipment, bedrolls and jerry cans etc. Drawings to make one for your self can be requested from Jeepdraw (free; see links-page)
See pic