I have discussed before on this and on my Aryan Myth and Metahistory blogs about the interrelatedness of the Northwest Indo-Eurupean Thunder Gods and a possible connection with the Indo-Aryan Parjanya, a much older deity that appears to have been usurped by Indra by the time that the Rig Veda (originally an oral work) was written down. http://aryan-myth-and-metahistory.blogspot.co.uk/2014/10/parjanya-original-indra-and-cognate-to.html
I have already demonstrated that the Germanic *Thunaraz, the Celtic Taranis, the Lithuanian Perkunas, the Latvian Perkons, the Prussian Perkonis, the Russian Pyerun, the Czech Perun are all descended from the PIE *Perkwunos/*Perkunos. The etymological relationship with the PIE form is more obvious from the Baltic and Slavic names for this deity but a relationship with * Thunaraz and indirectly with Taranis may be found through the name of an obscure Germanic deity Fjorgynn who is the father of the Goddess Frigg and is mentioned only twice in the Eddas:
"Be silent, Frigg, you're Fiorgyn's daughter and you've always been mad for men: Ve and Vili, Vidrir's wife, both were taken into your embrace." ( Lokasenna 26, Elder Edda, Larrington translation).
"Be thou silent, Frigg! Thou art Fiorgyn's daughter, and ever hast been fond of men, since Ve and Vili, it is said, thou, Vidrir's wife, didst both to thy bosom take." (Thorpe translation)
Vidrir it should be noted is just a heiti, a by-name for Odin, meaning 'weather god'. The incident of Frigg's unfaithfulness is recounted more fully in Snorri's Ynglinga Saga. Incidentally Frigg's conduct is more reminiscent of Freyja's character and I have argued before that these are just two aspects of a primordial Germanic Goddess. http://celto-germanic.blogspot.co.uk/2014/08/frigg-and-freyja-originally-same-deity.html
The other reference to Fjorgynn as Frigg's father is found in Skaldskaparmal in the Younger Edda:
"How shall Frigg be referred to? By calling her daughter of Fiorgyn, wife of Odin, mother of Baldr, rival of Rind and Gunnlod and Gerd, mother-in-law of Nanna, queen of Aesir and Asyniur, of Fulla and falcon form and Fensalir." (Faulkes translation)
Fjorgynn (the correct spelling of Frigg's father's name) is not to be confused with Fjorgyn which is a by-name for Frigg and means earth. Jacob Grimm connects Fiorgynn with the Thunder God:
"The neut. noun fairguni (Gramm. 2, 175. 4530 means mountain. What if it were once especially the Thunder-mountain, and a lost Fairguns the name of the god (see Suppl.)? Or, starting with fairguni with its simple meaning of mons unaltered, may we not put into that masc. Fairguns or Fairguneis, and consequently into Perkunas, the sense of the abovementioned, he of the mountain top? a fitting surname for the thundergod.
"Now it is true that all of the Anzeis, all the Aesir are enthroned on mountains (p.25), and Firgun might have been used of more than one of them; but that we have a right to claim it specially for Donar and his mother, is shown by Perun, Perkun, and will be confirmed presently by the meaning of the mount and rock which lies in the word hamar."(Teutonic Mythology Volume 1)
At this point it may be useful to remember that the Proto-Germanic *Thunaraz and the PIE *Pekwunos are etymologically related to the name of the Thunder God amongst the non-Indo-European Finno-Ugric peoples, most especially the Estonians (Turris, Peko and Pekolaso). The Sami Thunder God is Horagelles. Whether or not the non-PIE examples point to cultural borrowing or to a common pre-Aryan/pre Finno-Ugric origin is difficult to tell but by studying other Indo-European and indeed non-Indo-European but northern European mythologies we can learn much more about *Thunaraz than is revealed in just the Eddas and saga material. http://celto-germanic.blogspot.co.uk/2013/08/finnish-and-estonian-thunder-gods.html
The image at the head of this article is a photograph of reportedly the oldest surviving oak tree in Europe, dating back between 1500-2000 years in Stelmuze in Lithuania. Perkunas was worshipped under this tree.