If you are getting a WWAN card for the ThinkPad X1 Carbon Gen 2, make sure you have the WWAN antenna installed in the machine already. Otherwise, you would have to get the WWAN and WIFI antenna kit and install it yourself, which involves disassembling the whole machine.
The best way to check if you have the WWAN installed, is to remove the base cover of the ThinkPad X1 Carbon Gen 2, which is held down by 7 screws located on the bottom of the machine. The instruction on how to remove the base cover, and other upgrades and repair instructions can be found in the link below.
ThinkPad X1 Carbon Gen 2 Hardware Maintenance Manual
Once you open up the ThinkPad X1 Carbon Gen 2, you should see the WWAN antennas (which is not attached to anything) are in the upper right corner of the machine (near the wireless card). If you don’t have the WWAN antenna kit, then you need to either reconsider your plan to put WWAN card in your machine, or get the WWAN and WIFI antennas kit to install it yourself (but you may void your warranty by doing so).
The WWAN and WIFI antenna kit can be ordered from Lenovo with the following parts number:
FRU 0HM107 Antenna Kit WWAN + WLAN
For the WWAN card, it must be a Lenovo whitelisted version with the following FRU number:
04X6014 Sierra EM7345 LTE/HSPA+ 42Mbps M.2 Card
04X6038 Sierra Gobi500 EM735 LTE/EVDO/HSPA+ 42Mbps M.2 Card
04W3842 Ericcson N5321 HSPA+ 21Mbps M.2 WWAN
The WWAN slot on the ThinkPad X1 Carbon Gen 2 is a M.2. slot with B connector type.
When you have the right WWAN card from the parts list, it is quite easy to install.
One point to note, the orange wire should be attached to the main antenna nodule on the WWAN card, and the blue goes with the AUX antenna nodule on the card. The WWAN card has the corresponding antenna colour codes listed next to MAIN and AUX, so if you get lost just look at the colour code.