Jacques;

General: Refer to: http://www.sw-em.com/1800%20Wiring%20Diagram.jpg While troubleshooting electrical problems on any Volvo, harness color code is your friend!

Horns are a Battery Powered load (not Ignition Powered) and are on a completely separate circuit from Taillights. After assuring Horns are getting power but still not sounding, either replace, or open and repair...see: http://www.sw-em.com/122_horn_notes.htm

Taillights are separately fused in the 1800...as a matter of fact they are THE ONLY thing that is separately fused in an 1800...everything else is on either the Battery Power Fuse OR the Ignition Power Fuse...but Taillights are fused by the SINGLE Fuse3...if this is the one which keeps blowing, that certainly explains why you have no Taillights...you likely have a short on that circuit which you need to locate and remedy.

Additional: All fuseholders in the 1800 are Lucas electrical aberrations and should be replaced! ...even in the low humidity conditions of California, at this age I would consider them highly suspect...check for resistance heating! See: http://www.sw-em.com/tech_bulletin_3.htm

Good Hunting!