Any idea what error code 18 means in a 1981 Cadillac DeVille?

SOURCE: Check Engine light

that code definition is "Evap high purge flow fault". It means you may have a problem with the evaporative system. this system routes fuel fumes back to the fuel tank. If you want to try and clear this code, find the radio code for your car and then disconnect the battery for 30 seconds. Reconnect the battery and the check engine light should be out. You will then need the radio code to reset your car stereo. the check engine light may stay off if the condition was a transient one. If not, the light will come back on after you drive the car two drive cycles. That is: two starts, warm ups and shut downs. If the light returns, have a mechanic check it out.

SOURCE: 2003 Cadillac Deville a check gas cap error appears with svc engi

put a new gas cap, those system are pres. even if
the cap look ok it maby leaking replace with a new
one it should solve your prob.
eng.ligh come on because of the gas cap

SOURCE: check enginie light is on code that comes up is p1698 i have no idea what that means and my car seems to be ok

Here is my "take" on the P1698 code:
The P1698 code means that no digital data is being received by the powertrain controller from the transmission control module, located on the right inner fender. The manual says to check the ccd bus wires between the module and the rest of the system using the Chrysler diagnostic readout box. But if you don't have one then I think you can do the same thing by measuring for continuity (near 0 resistance) between pin 4 of its plug and pin 11 of the data link plug under the dash or pin 59 of the powertrain controller plug.
Then check the resistance between pin 43 if its plug and pin 3 of the data link connector or pin 60 of the powertrain controller plug. The powertrain controller is on the opposite side of the engine compartment on the left inner fender. These module plugs are held in place by a bolt. If the continuity between the transmission and the powertrain CCD wires checks to be ok, then according to the manual the trans controller should be replaced. You may be smart however to find an honest shop that has a code reader and let them determine for sure that it is the TCM and not the Powertrain control module (PCM) that is the fault. These modules are expensive enough that you would not want to replace the wrong one.
The other symptoms (1-3) are likely related to the problem because they are also dependent on the digital data communication bus for getting their instructions or sending their information. The passenger side window is probably not related to this, but rather a failed connection, switch or motor.

SOURCE: Cadillac Deville 2000- Engine check light code P0741

If you like go ahead and let someone sell and replace your transmission but here is my suggestion! Check around and see if you can find a minor vacuum leak! This can simply be the big round hose that goes from the fuel injection to the power brake hub! This is normally a push in connector which has about a 1/2 inch hose clamped on it! If there is any kink or the connector is not in correctly a leak will occur and the transmission will shift rough as though you will be thrown out of the vehicle! Try unplugging this connector and plug it back in hard and straight and maybe you will be lucky and fix your problem. If the vacuum is correct your transmission will shift better than you have ever experienced!

SOURCE: Cadillac 1999 DeVille Battery goes dead after

You should check for voltage draws when the vehicle is off... Disconnect the negative battery termal, Pull dome light fuse,and connect a light or volt meter between the two.

common offenders are alternator diodes. These fail and cause drain.

Then you have to check, circuit by circuit, until you find the offending system.

Also, before doing anything, disconnect negative terminal and see if the battery is still fully charged after a few days...