My truck is a 1998 lincoln navigator, signal lights work sometime

SOURCE: 1998 Lincoln Town Car

If your main brake lights are not working and the third light in the back window is working
it is the turn signal switch on the column cost about 150 to have replaced

SOURCE: Need help w/1998 Navigator

Coil Pack or coil packs are bad there is 1 for each spark plug

SOURCE: where are the plugs on lincoln navigator 1998

They are under the coil packs. You will need an 8mm socket to remove the bolt on each coil pack then remove them to access spark plugs.

SOURCE: front and rear turn signals not working on lincoln navigator

sounds like
turn signal relay

SOURCE: my 1998 navigator 4 wheel drive got stuck& now ac wont work

2 different electrical circut's that are unrelated. Ac on a 12 year old truck is bound to need some service. Take it to a trusted shop that is not hurting for work & ask them if anything is disconnected, ie: compressor power wire, high pressure switch...ect. Tell tale sign system is low on freon is the compressor keeps cycling on & off with a clicking sound or compressor does not come on at all.

Testimonial: "Thanks that relieves my mind that they might have disconnected something accidently. "